Shell and tube heat exchangers have an array of tubes extending between and through two spaced apart tube sheets surrounded by a shell. The shell is provided with an inlet and an outlet so that a suitable heat exchange liquid or gas can be circulated through the shell to cool or heat a liquid flowing through each tube.
Each end of the array of tubes can be left open or exposed, for use in some processing operations. For other operations, one or both ends can be enclosed by a liquid retaining header, which may or may not have a removable cover or access port.
Although shell and tube heat exchangers are generally used to heat a liquid feed stream, they can be used for cooling such a stream. Shell and tube heat exchangers of the described types can be used as freeze exchangers for producing fresh water from brackish water and seawater, for concentrating fruit and vegetable juices, coffee and beer, and industrial crystallization processes. As the liquid flows through each tube, it can be cooled enough to crystallize a solid from the liquid. Thus, by cooling seawater, ice is obtained which when separated, washed and melted provides potable water. When a fruit or vegetable juice is similarly chilled, ice forms and is removed to provide a concentrated juice.
Freeze exchangers of the described type can use any cooling fluid on the shell side to cool a liquid flowing downwardly through the tubes. The cooling fluid can be fed through one end and removed through the other end of the freeze exchanger in a substantially unidirectional flow. Some suitable cooling fluids include refrigerant gases such as ammonia and Freon brand refrigerants, and aqueous ethylene glycol.
When a refrigerant is used, it is generally liquefied first and then fed as a liquid to the shell side of the freeze exchanger through an inlet port in the lower part of the shell. A volume of refrigerant liquid is maintained on the shell side with a refrigerant vapor volume above the liquid level. The refrigerant liquid level is usually maintained about at the lower part of an outlet port in the shell below the upper tube sheet. Accordingly, essentially refrigerant vapor and little liquid is withdrawn from the outlet port.
Even though the cooling liquid essentially fills the shell side around the tubes, heat exchange across the tubes is unexpectedly lower than one might expect. This is in part, perhaps, due to a somewhat stagnant liquid pool around the exterior surfaces of the tubes and a tendency for vapor to cling to that surface. A need accordingly exists for improving heat exchange between the liquid in the tubes and the heat exchange fluid outside the tubes.